Inflow is one of the most important basis for reservoir scheduling. The accuracy of the forecast for the progression of mainstream flow between adjacent reservoirs in a cascade reservoir group and the inflow process of the inter-basin area is of great significance to improving the overall scheduling level of cascade hydropower stations and fully utilizing the efficiency of water energy resources in the basin. In response to the lack of data between each reservoir in the cascade, the inflow process of non-leader reservoirs in the cascade is divided into two parts: "outflow from the upper reservoir" and "inflow from the inter-basin area". The outflow from the upper reservoir is derived using the Muskingum flow routing method to the lower reservoir. For the inflow from the inter-basin area, a correction coefficient is first introduced to correct the inter-basin flow process obtained by retrogression based on water balance and flow lag, and then it is divided into two parts: inter-basin rainfall runoff and inter-basin river base flow. The inter-basin rainfall runoff is calculated using the rainfall-runoff relationship curve and the unit hydrograph method, and the inter-basin river base flow is obtained using the linear segmentation method. The practical application in the cascade reservoir group in the middle and lower reaches of the Hongshui River shows that the inflow process predicted by this method has been improved in several indicators such as NSE, variance, correlation coefficient, and coefficient of determination compared to the uncorrected traditional calculation method, demonstrating the stability and accuracy of this inflow forecasting method. The research results can provide a reference for cascade hydropower stations to carry out short-term scheduling and real-time scheduling.